Ink jet printers are well known in the art. Generally, an ink jet printer includes an array of nozzles or orifices, a supply of ink, a plurality of ejection elements (typically either expanding vapor bubble elements or piezoelectric transducer elements) corresponding to the array of nozzles and suitable driver electronics for controlling the ejection elements. Typically, the array of nozzles and the ejection elements along with their associated components are referred to as a print head. It is the activation of the ejection elements that causes drops of ink to be expelled from the nozzles. The ink ejected in this manner forms drops which travel along a flight path until they reach a print medium such as a sheet of paper, overhead transparency, envelope or the like. Once they reach the print medium, the drops dry and collectively form a print image. Typically, the ejection elements are selectively activated or energized as relative movement is provided between the print head and the print medium so that a predetermined or desired print image is achieved.
Generally, the array of nozzles, supply of ink, plurality of ejection elements and driver electronics are packaged into an ink jet cartridge. In turn, the printer includes a carriage assembly for detachably mounting the ink jet cartridge thereto. In this manner, a fresh ink jet cartridge may be installed when the ink supply of the current ink cartridge has been consumed. Some ink jet printers provide an indication to the user that the ink supply is running low while others do not. In either case, the printer continues to operate with the result being that the user must recognize when the ink supply is exhausted. Thus, it is generally intended for the ink jet cartridges to be disposable.
Recently, the postage meter industry and other envelope printing industries have begun to incorporate ink jet printers. A typical postage meter (one example of a postage printing apparatus) applies evidence of postage, commonly referred to as a postal indicia, to an envelope or other mailpiece and accounts for the value of the postage dispensed. As is well known, postage meters include an ascending register, that stores a running total of all postage dispensed by the meter, and a descending register, that holds the remaining amount of postage credited to the meter and that is reduced by the amount of postage dispensed during a transaction. Because U.S. Postal Service regulations require that postage be paid in advance, it had traditionally been required that the user of a postage meter periodically present the meter to a Postal Service employee for recharging. However, more recently it is possible to recharge a meter remotely using telephone communications. At the time of recharging, the user paid to the Postal Service the amount of postage to be credited to the meter and the meter is recharged by increasing the setting of the descending register by the amount paid. The postage meter generally also includes a control sum register which provides a check upon the descending and ascending registers. The control sum register has a running account of the total funds being added into the meter. The control sum register must always correspond with the summed readings of the ascending and descending registers. The control sum register is the total amount of postage ever put into the machine and it is alterable only when adding funds to the meter. In this manner, the dispensing of postal funds may be accurately tracked and recorded.
Due to the inherent nature of printing an indicia of value (a postal indicia being the equivalent of money), several issues arise with utilizing ink jet printing in a postage printing device. For example, if a general purpose ink jet printer runs out of ink while printing a document, then the user merely installs a new cartridge and reprints the document. On the other hand, if a postage printing device runs out of ink while printing a postal indicia, then the user loses money because the postal funds associated with that postal indicia cannot be recovered. Therefore, it is desirable to avoid running out of ink while printing a postal indicia.
Complicating this issue is the practical effect that current techniques for determining how much ink remains in the ink jet cartridge suffer from various drawbacks. Passive techniques estimate the amount of ink that remains by counting the drops that are ejected and subtracting a given volume of ink for each drop from the supply of ink. However, the drop volumes vary according to the operating conditions such as: temperature, humidity, and variable sensitivity of the ejection elements to the drive signals between different ink jet cartridges and over the life of the cartridge. Thus, these techniques are only somewhat accurate. Active techniques estimate the amount of ink that remains by physical measurement of the supply. This may be accomplished by a variety of known methods such as employing a thermistor to detect the changing level of the ink supply in the reservoir. However, these techniques add to the hardware complexity and cost of the ink jet cartridge. Additionally, these techniques are also only accurate to within a certain range. For example, inaccuracies in measuring the ink remaining are introduced due to a non-level printer module, excessive vibrations, calibration errors and the like.
Furthermore, there are no assurances that the cartridges were full when they were installed. Thus, the assumption that brand new cartridges are installed each time may not be correct resulting in further inaccuracies.
Although on the one hand, operators want to avoid running out of ink while printing a postal indicia so as not to lose postal funds; on the other hand, operators want to print as many postal indicias per cartridge as possible so as to lower their operating costs. Thus, these two objectives are in conflict. Running out of ink is undesirable because of the lost customer postal funds. Disposing of cartridges while they still have usable ink remaining is undesirable because the remaining ink has been paid for but never used resulting in an increase in the cost of consumable supplies or cost per postal indicia dispensed.
Therefore, there is a need for a postage printing apparatus that prevents printing a postal indicia if an out of ink condition is near, overcomes the inaccuracies of passive measurement techniques and does not introduce the cost of active measurement techniques. In this way, the cost of the ink jet cartridge is spread over as many envelopes as possible and the overall cost of the postage printing apparatus is reduced.